1. Field of the Invention
The present invention relates to an electrostatic chuck device, more particularly to an electrostatic chuck device for chucking and fixing a substrate in a substrate processing chamber in a semiconductor production process.
2. Description of the Related Art
In recent years, in processing substrates or wafers such as the formation or deposition of a thin film on a substrate (sputtering, chemical vapor deposition (CVD), etc.) or dry etching in a semiconductor production process, electrostatic chuck devices are frequently used for fixing the substrates or wafers onto a wafer holder. In comparison with the conventional mechanical clamping units, when fixing a substrate, an electrostatic chuck mechanism does not have any parts for touching and holding the upper surface of the substrate. Accordingly, a percentage of devices obtained from one substrate becomes high and a yield rate of the devices can be raised. Further, precise control of the temperature through a combination with a temperature regulator becomes possible.
Electrostatic chuck (ESC) devices are divided into two types. One is a unipolar type and another is a bipolar type. Operation of a unipolar device requires plasma over the entire surface of the substrate. This plasma creates the electrical connection required for generating an electrostatic force. A bipolar device, however, operates even without plasma. Therefore, generally, the electrostatic chuck devices can be used for both plasma process and non-plasma process.
An example of a conventional electrostatic chuck device will be explained with reference to FIG. 23. This electrostatic chuck device is applied to a sputtering system, for example. In this sputtering system, the pressure inside of a metal vessel 11 is reduced in pressure by an evacuation mechanism (not shown). A disk-shaped target 13 supported by a ring-shaped insulating member 12 is attached to the ceiling part of the vessel 11. In the outside of the vessel 11, magnets 15 fixed to a yoke 14 are placed at the back of the target 13. At the lower section of the inside of the vessel 11, a substrate support 16 is provided. A substrate 17 is loaded on the top surface of the substrate support 16. The substrate 17 is arranged to face the target 13 in parallel to it. The substrate support 16 is fixed to the bottom of the vessel 11. The substrate support 16 is provided with an electrostatic chuck device 18 and a substrate temperature regulator 19. A cylindrical shield 20 is provided near the inside surface of the surrounding walls of the vessel 11.
The electrostatic chuck device 18 is comprised of a dielectric plate 22 on the surface of which an embossed part 21 is formed by an embossing process, and a metal electrode 23 arranged at the inside of the dielectric plate 22. The height of the embossed part is 5 to 25 μm. The metal electrode 23 has, for example, a bipolar electrode structure comprised of an inside electrode 23a and outside electrode 23b. The metal electrode 23 is connected to an external DC power circuit 24 and supplied with a certain voltage. The external DC power circuit 24 includes a battery 24a for supplying a plus voltage, a battery 24b for supplying a minus voltage, a ground terminal 24c, and switches 24d and 24e. The inside electrode 23a is supplied with a plus voltage by the battery 24a, while the outside battery 23b is supplied with a minus voltage by the battery 23b, for example. The substrate (silicon substrate) 17 is fixed on the substrate support 16 by the coulomb force (electrostatic force) acting between the metal electrode 23 and the substrate 17 when it is placed on the dielectric plate 22 and a predetermined voltage is supplied to the metal electrode 23. The coulomb force is generated by the charges induced at the surface of the dielectric plate 22. The substrate 17 is clamped on the surface of the dielectric plate 22 by the coulomb force.
The substrate temperature regulator 19 is provided below the electrostatic chuck device 18. The substrate temperature regulator 19 is comprised of a thermocouple 25, a power source/control mechanism 26, and a heating/cooling unit 27. The cooling/heating unit 27 is controlled to a required predetermined temperature by the thermocouple 25 and power source/control mechanism 26 and holds the dielectric plate 22 provided on it at a predetermined temperature. The temperature of the substrate 17 on the embossed part 21 of the dielectric plate 22 is held at a predetermined temperature by heat conduction of a gas introduced into the clearances 21a formed by the embossed part 21 by a gas supply source 28 and gas introduction path 29, and the clearances 21a are held at a predetermined pressure.
When the substrate 17 is clamped by the electrostatic chuck device 18, the clamping force has to be sufficiently larger than the force due to the differential pressure between the pressure in the clearances 21a and the internal pressure of the vessel 11. The normal sputtering pressure is several milli-torrs (mTorr). Therefore, the value of the differential pressure is substantially equal to the pressure of the clearances 21a. In this case, the value of the differential pressure is about 10 Torr.
The force clamping the substrate 17 is the coulomb force acting between the surface of the dielectric plate 22 and the substrate 17. The coulomb force will be explained with reference to FIG. 24 and FIG. 25.
The substrate 17 is placed on the embossed part 21 of the dielectric plate 22. As shown in FIG. 24, when a plus voltage is supplied to the metal electrode 23 (inside electrode 23a) from the battery 24, plus charges are induced on the surface of the dielectric plate 22, and simultaneously minus charges are induced on the back of the substrate 17.
When the unipolar electrode is used as the metal electrode, the substrate 17 is electrically connected to the ground for the power source through the plasma and a closed circuit is formed. When the bipolar electrode is used as the metal electrode, charges appear at the back of the dielectric plate 22 corresponding to each of plus and minus voltages of the internal and external electrodes. A closed circuit is formed through the back surface of the substrate 17, and the charges are induced on the back surface of the substrate 17. Due to the formation of the closed-circuit for a DC current via the back surface of the substrate, the substrate gets electrostatically chucked without a plasma.
The force (F) acting between the surface of the dielectric plate 22 and the substrate 17 satisfies F=∈(V2/L2)A/2 in the case of the unipolar electrode, while it satisfies F=∈(V2/L2)A/8 in the case of the bipolar electrode with the same plus and minus electrode areas. Here, ∈ is the dielectric constant of the clearances 21a, V is the voltage, L is the distance between the dielectric plate and the back of the substrate (back of silicon substrate), and A is the electrode area. The clamping force is proportional to the supplied voltage and electrode area and is inversely proportional to the distance between the substrate and the dielectric plate. In sputtering, it is necessary to heat the substrate before film formation and to hold it at a predetermined temperature, so usually the bipolar electrode is used.
On the other hand, at projections 21b of the embossed part 21 where the substrate 17 and the dielectric plate 22 come into direct contact, as shown in FIG. 25, fine clearances 30 (distance δ of clearances) are generated due to the fine projections and recesses on the surface of the substrate 17 or dielectric plate 22. The distance δ of the clearances 30 is extremely small or about 0.1 μm, so the force generated across the clearances 30 becomes extremely large. This is called the “Johnsen-Rahbek effect” (“JR effect”).
The clamping force will be calculated for the case of the bipolar electrode. When processing a substrate whose diameter is 300 mm, it is assumed that the diameter of the dielectric plate 22 is 300 mm, and the outer periphery with 1 mm of the dielectric plate 22 and the embossed projections contact the substrate and support it. The surface area of the contact sections becomes 1% of the horizontal area of the entire dielectric plate. As the metal electrode 23, a bipolar electrode split into two to form an inside circle and an outside ring is used. The metal electrode 23, for calculation purposes, is assumed to be a disk with a diameter of 298 mm. The voltage supplied to the metal electrode is made +200 V for the plus electrode and −200 V for the minus electrode. The emboss gap (difference in height between the projections and recesses) corresponding to the distance L between the substrate 17 and the dielectric plate 22 is 7 μm, while the fine clearance δ of the contact parts between the embossed projections 21b and the substrate is 0.1 μm, for example. Further, the clamping force is assumed to act on the surface of the dielectric plate in only the vertical direction.
The force acting on the depressions of the embossed part 21 becomes 500 N to 600 N and the force acting on the projections 21b becomes 5000 N to 10000 N. Therefore, the whole force on the embossed part 21 becomes 5500 N to 10600 N. The force acting on the projections 21b is extremely large and important in control. The total of these forces acts on the substrate as a whole, but the force per unit area, or the pressure, becomes 500 Torr to 1000 Torr. This pressure is sufficiently larger than the force due to the differential pressure between the pressure of the clearances between the substrate 17 and the dielectric plate 22 and the internal pressure of the vessel 11, so the substrate 17 is stably clamped and fixed on the dielectric plate 22.
Next, the sputtering process for the substrate 17 in the vessel 11 of the sputtering system will be explained.
The substrate 17 is carried into the vessel 11 and placed on the dielectric plate 22 of the substrate support 16. The substrate 17 is conveyed by a not shown conveyance robot and lift pins. Next, the external DC power circuit 24 is operated and predetermined voltages are supplied to the electrode 23. In this example, +200 V is supplied to the internal electrode 23a, while −200 V is supplied to the external electrode 23b. The internal electrode 23a and the external electrode 23b are supplied with the same voltages in absolute value. When the electrode 23 is supplied with voltages, as explained above, the electrostatic force clamps and fixes the substrate 17 to the dielectric plate 22. When the substrate 17 is fixed, a gas is introduced to the clearances 21a formed between the substrate 17 and the dielectric plate 22 from a gas supply source 28 through a gas introduction path 29. The pressure within the clearances 21a is controlled to a certain predetermined pressure in the range of 1 Torr to 10 Torr. Due to this gas, heat is conducted from the dielectric plate 22 held at a predetermined temperature by the substrate temperature regulator 19 to the substrate 17. As a result, the substrate 17 also rises in temperature and is held at a predetermined temperature. When the substrate temperature reaches a certain level, Ar gas is introduced into the vessel 11 and the pressure within the vessel 11 is held at a predetermined pressure. Next, the target 13 facing the substrate 17 is supplied with a high voltage from a sputter power source 31, electric discharge occurs within the vessel 11, and a desired thin film is formed on the substrate 17 by the sputtering action on the target 13. After the formation of the film ends, the introduction of gas into the inside of the vessel and the supply of gas into the clearances 21a are stopped. After the pressure sufficiently falls, the supply of voltage to the electrode 23 is stopped. Next, not shown lift pins are used to separate the substrate 17 from the embossed part 21 of the dielectric plate 22 and a conveyance robot which is similarly not shown is used to convey the substrate 17 out of the vessel 11.
According to the above configuration of the conventional electrostatic chuck device, two important problems of the generation of particles and declamping of the substrate explained below arise.
Problem of generation of particles: The conventional electrostatic chuck device is set so that the clamping force between the substrate 17 and the embossed part 21 of the dielectric plate 22 becomes strong. Therefore, as problems, it has arisen that at the time of start of clamping, the back of the substrate 17 rubs against the dielectric plate 22 and the substrate 17 is abraded, and accordingly large amounts of particles are generated and become sources of dust causing a drop in yield.
As shown in FIG. 25, the substrate 17 and the top surfaces of the projections 21b of the embossed part 21 at the dielectric plate 22 come into contact at several points. These contact points form clearances 30 between the substrate 17 and the projections 21b. The clearance 30 is in a vacuum state or filled with an inert gas. Therefore, in calculating the electrostatic force, a large clamping force is generated by the distance of “δ” shown in FIG. 25. This means that the substrate is basically fixed on the electrostatic chuck device 18 by the force generated on the embossed part. That is, at the back of the substrate 17, an extremely large pressure is present on a smaller surface area. As a result, the substrate 17 and the dielectric plate 22 are abraded by friction and fine particles are generated. Part of these particles directly sticks on the back of the substrate, while the remainder falls into the depressions (or clearances 21a) of the embossed part 21 and is deposited there. With repeated processing of a substrate 17, the number of particles deposited in the depressions increases and the particles start to stick on the back of the substrate. The particles stick on the back of the substrate for two reasons. The first reason is the electrostatic force generated between the substrate and the particles. The second reason is that the particles start floating freely due to the rapid flow of the inert gas through the clearances 21a between the substrate and the dielectric plate. These free-floating particles can stick on the back of the substrate.
To solve the problem of the generation of particles, it is sufficient to reduce the area of the parts of the projections 21b contacting the substrate. Since these parts have the function of supporting the flexing substrate, however, there are limits to the reduction of the area.
Problem of declamping of substrate: The charge given at the surface of the dielectric plate 22 remains even after stopping the supply of voltage to the metal electrode 23 after processing the substrate 17, so the clamping force does not immediately dissipate.
The problem of declamping will be explained considering the above-mentioned hardware configuration and formula for obtaining the electrostatic force. To declamp the substrate, the electrodes 23a and 23b of the metal electrode 23 are disconnected from the batteries 24a and 24b and connected to the ground by operating the switches 24d and 24e. 
When a DC voltage is given to the electrodes 23a and 23b, first charges build up on the electrodes 23a and 23b. These charges migrate slowly toward the dielectric plate 22, that is, the top surface of the embossed part 21, due to the presence of the strong electric field (E1: shown in FIG. 24) generated between the substrate 17 and the electrodes 23a and 23b. The migration of the charges to the top surface of the embossed part 21 is due to the fact that the dielectric plate 22 normally is not a perfect insulator. Further, the dielectric plate 22 is deliberately doped with an impurity to reduce the electrical resistance. Finally, the charges accumulated on the top surface of the dielectric plate 22 or the top surface of the embossed part 21.
On the microscopic scale, the lower surface of the substrate 17 and the top surfaces of the projections 21b of the embossed part 21 are rough. Actual contact between the substrate and the dielectric plate 22 occurs only at a few locations as shown by FIG. 25. Due to the accumulation of charges on the top surface of the dielectric plate 22 (the top surface of the embossed part 21), the above electrical field (E1) is reduced. Instead, the electrical field between the top surfaces of the projections 21b of the embossed part 21 and the substrate 17 becomes stronger. The charges generated on the top surface of the dielectric plate 22 fall along with the elapse of time. Therefore, even if no problem arises with respect to fixing the substrate by the electrostatic force, a problem arises in the operation to release the substrate which has been clamped. The reason is that when the electrodes 23a and 23b are connected to the ground to release the substrate, the charges on the top surface of the dielectric plate 22 will not immediately flow back to the electrodes 23a and 23b. Re-flow of the charges of the electrodes 23a and 23b depends on the electrical resistance of the dielectric plate 22. To facilitate the re-flow of the charges to the metal electrodes 23a and 23b, the dielectric plate 22 is doped with an impurity so as to reduce its electrical resistance. However, due to the re-flow of charges to the metal electrode, the electrical field in the dielectric plate 22 is weakened and therefore the re-flow of the charges gradually slows along with the elapse of time. In this way, the complete neutralization of the dielectric plate 22 by the charge re-flow process requires considerable time. Accordingly, swift release of the substrate cannot be readily achieved.
Therefore, if trying to separate the substrate 17 from the dielectric plate 22 by lift pins for the purpose of conveying the substrate 17 outside of the vessel 11, the substrate will generate a vibration and a deviation in position. As a result, the problems of deterioration of distribution of the charges and inability of conveyance etc. will occur in the later substrate processing, and further the problems of a drop in yield or a drop in system operating rate will be caused.
As a prior art related to the above problems, the electrostatic chuck device disclosed in Japanese Unexamined Patent Publication (Kokai) No. 11-251416 may be further mentioned. With this electrostatic chuck device, a good releasability of the clamped object is realized.
As further related art, U.S. Pat. No. 5,530,616 and J. Daviet, L. Peccoud, J. Electrochem. Soc., Electrochemical Association, November, 1993, vol. 140, No. 11, pp. 3251-3256 may be mentioned.